The fight is now in the federal courts over whether to blow the levee below Cairo in order to save the city. The area in question is the New Madrid Floodway and was designed to be flooded in emergencies.
Since the 1927 Mississippi River Flood that has only happened once, in 1937. The Corps of Engineers wants to open a gap while Missouri is fighting to block the flooding.
Here's the area in question that would be flooded. Basically, there is a levee along the Mississippi on the right side of the area and another levee protecting a larger section of communities on the left side of the designated area.
The map is my best estimate based on the existing levee roads. The area stretches from Bird's Point, Mo., opposite Cairo, to New Madrid.
View New Madrid Floodway in a larger map